I think it's the resolution that is most likely to be the reason to differences in "render times"!Usually my preview area/window is not more than 1/4 of my screen (very rarely full screen), and my final renders are from 1080, to 4K, to more...

At your prompt, I just stretched out my preview to about 1920 x1080 , taking most of the secondary monitor.

The preview is still really fast. About 3 seconds.

The Render, at 1280 x 720 is about 12 seconds.

Doesn't seem to make sense, does it?

What really puzzles me: I don't see the lag from mesh compiling that I see in the actual render. After the initial compilation, the render just snaps into place as I move on the timeline.This is what I want for test renders!